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Post subject: Simulating Amp Mods with LT spice (or other free product)
Posted: Thu Feb 07, 2013 6:05 pm
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OK

Starting this thread to do exactly what the subject says. And doing this in software "might" save a life, Keep the little woman happy (no smoke,) and in the case of newer amps, it will probably make the PCB last a little longer. :)

Shimmi,

Can you repost that screen shot and let me know what that V2 is? It certainly is NOT a tube.

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 6:16 pm
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I'm just here for the ride. :wink:

...trying to learn something new too.

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 6:21 pm
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Welcome aboard.

I think it is Variable signal but I'll let Shimmi explain that. I learned my electronics back in the 70's. We didn't have this newfangled software. LOL, we didn't have software, period. :lol: Unless the girlfriend counts. :evil:

Art


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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 9:34 pm
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Thanks for starting this thread Art1. :D Apologies to bluesky636 for the hijack of an otherwise fine thread.

OK, I thought that I would start by copying some of the posts from the other thread.

Have you tried "Spice"? I use LT Spice IV, and found several tubes to add to the library, albeit from various sources. You can make your own models of virtually any component via the instructions, but I find it easier to locate existing models and copy them to my library.

Just a couple of sites to get you started. It can be complicated and time consuming to make your own tube models, for me at least.

http://www.linear.com/designtools/software/#LTspice

http://www.duncanamps.com/technical/ltspice.html

It is a fairly intuitive program, if you have some experience with schematics and circuits in general. It's pretty much drag-and-drop, or click "Open", and connect-the-dots. The main trouble is just finding tube models already made, or making your own. I actually started many years ago with "Electronics Workbench" and "Multisim", which are both very similar to the Spice programs, and only fairly recently started using Spice.

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 9:45 pm
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Art1 wrote:
...
Shimmi,

Can you repost that screen shot and let me know what that V2 is? It certainly is NOT a tube.

Art


The "V2" in the circuit below, as you thought, is an AC supply. This circuit is one that I made, copying the +/- 16 VDC supply from the HRDlx schematic. I used it to simulate what happens when changing the resistance values of R78 and R79 from the stock 470 ohms to 330 ohms as some people wrongly suggest as some sort of supposed fix for the heat in this circuit.

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 9:56 pm
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shimmilou wrote:
Thanks for starting this thread Art1. :D Apologies to bluesky636 for the hijack of an otherwise fine thread.


No problem. :wink:

I really think this is an excellent topic, one that I want to learn more about. :D Plus, its not another "How do I bias a (name your amp)." :lol:

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 10:40 pm
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To start a new circuit for simulation, open the program and click the icon on the far left "New Schematic". As you can see, many standard components like resistors, capacitors and diodes are available on the main toolbar, and you simply click on any component that you want to place in your circuit, and then click on the schematic at every place that you want this particular component to be placed. Click as many times for as many of that particular component as needed for your circuit, each placed component will be labeled in order, R1, R2, R3, etc. Components and other commands can also be selected from the menus, such as under "Edit". Once done placing that component, press "Esc" on your keyboard and the component on hold waiting to be placed will disappear, then you can choose your next component for placement. After placing components, you can right click on the placed component and assign a value. Right click on the component label to change to whatever name that you want, as in changing R1 and R2 to R78 and R79. Components can be moved as needed by selecting the "Move" icon on the toolbar, then dragging a box around the part(s) that you want to move. While selected for moving, you can select "Rotate" on the toolbar. You can also move the label(s) separately if you want, by dragging a box around only the label(s).

An example of three resistors placed, with one on hold waiting to be placed, simply click again to place a fourth resistor, or "Esc" if done placing.
Image

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Thu Feb 07, 2013 11:18 pm
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Once all components are placed (or whenever you want), click the "Wire" icon on the toolbar (looks like a pencil), and line up the cross hairs where you want to start and click at each point to draw wires connecting components. Start by clicking on one end of a component (small box), then click where you want a 90 degree turn, and another 90 if needed, then click on the end of the next component, etc. At any time if you make a mistake, click "Undo" under the "Edit" menu, and then reselect the "Wire" icon and continue. Click "Esc" on keyboard when done connecting. Other components can be selected by clicking the "Component" icon toward the right end of the toolbar (looks like an "And Gate"), and selecting from the library. This is where you find power supplies like a voltage or current source, and many other components. Add new components to your library from the internet, or even make and save your own. All placed components on your schematic can be assigned various values and tolerances by right clicking the component on the schematic. You can also select the "Open" icon from the toolbar and select more specific components from your library, like various values of Zener diodes, or even whole circuits to add to your existing schematic. Use the "Zoom" icons on the toolbar as needed to re-size your schematic.

Image

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 12:17 am
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When you have a completed circuit, with supply and components, you are ready for simulation. You can simulate with and without a load. Select the "Run" icon on the toolbar (looks like a person running), and a black box appears above your circuit. At this time you can move your cursor to any point on the circuit that you want to measure. Note than when the cursor is directly on a component, a current meter will appear, and when the cursor is on a wire, a meter probe will appear. Click when the meter or probe appear, in order to see the waveform at that particular point on the circuit, whether current or voltage is desired. Although it doesn't show in the screen shot, I moved the cursor over R78 of the HRDlx power supply circuit to view the current through the resistor. After clicking the component, the current waveform appears in the black box. Click on the waveform title at the top [ I(R78) ] to see the current value at a specific point in time, displayed in the box at the lower right. Move the crosshairs on the waveform by clicking on it and dragging to the desired point on the waveform, the values will be displayed in the lower right box. As you can see, the current settles at approx 45 mA at the right of the display as indicated in the lower right box. The voltage supply (V2) is a sine, 0 VDC offset, 40 volt amplitude, 60 hz, as displayed beside the source (the "+" and "-" are for phasing, it's still an AC voltage source). I selected these values to give me approx the same values indicated on the HRDlx supply schematic.

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Last edited by shimmilou on Fri Feb 08, 2013 1:27 am, edited 1 time in total.

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 12:39 am
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In this shot, while in run mode, I clicked on resistor R78 to view the current through the resistor, and clicked the wire on top of Zener CR13 to see the voltage output from the Zener. Clicking the title of the current waveform [ I(R78) ], and double clicking the title of the voltage waveform [ V(n003) ], gives the values for each in the box at the lower right. Moving the crosshairs for each waveform to the desired point on the waveform shows approx 45 mA current through R78, and approx 16 VDC out of Zener CR13.

Image

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 1:22 am
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Now, the same circuit with the only difference being R78 and R79 values changed to 330 ohms. Note there is still approx 16 volts from the Zener, but now the current through the resistor has increased to approx 62 mA. If you measure the voltage drop across, and current through R78 and R79, and compare the numbers between the 330 ohm and 470 ohm resistors, you can see that there is now even more wattage dissipated by the 330 ohm resistors than by the 470 ohm resistors. Using the 330 ohm resistors means that the Zeners also have increased current through them, which means more heat from the Zeners as well as more heat from the resistors (Ohm's law really works!). As you can imagine, using the simulator program is much easier and faster than swapping the actual components in the amp. :)

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 1:35 am
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Try some circuits for yourselves, or feel free to duplicate any of mine (not really mine per se), or come up with your own and post some screen shots.

Is anyone ready to see a tube circuit? :D

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 12:40 pm
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shimmilou wrote:
Is anyone ready to see a tube circuit? :D



The whole reason I'm doing this is for the "vintage" 60's Bassman circuits. :D

Art


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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 3:11 pm
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Can't wait to see it, post a screen shot when you have it completed. 8) So far I have some 12AX7, 12AU7, 12AT7, EL84 and EL34, but no 6L6 yet.

In the meantime, I downloaded this circuit already complete. This circuit has EL34s but it might help to get you started.

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Post subject: Re: Simulating Amp Mods with LT spice (or other free product
Posted: Fri Feb 08, 2013 3:41 pm
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So, I suspect in the case of a guitar amp, replace the input jack with an AC supply, or feed the input jack with an AC supply. As in a signal generator.

A 6L6 is just a pentode(Phillips had already patented the Power Pentode so RCA called it a "Beam Power Tetrode IIRC), but I guess in LT Spice we need to put in all of the electrical characteristics of the pentode. I'll have to see how hard it is to take an existing component and modify it. I think a 6V6 is basically the same with a lot less power.

So maybe we can come up with some components unique to our application? Swap out components in the tone stack and get a visual of what is happening. Of course it will never replace our ears.

Art


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